The present invention relates to the conversion of binary digital signals having definite bit transmission rates into pseudoternary A.C. pulses.
In many situations in communications technology, transmitting and receiving stations are connected together by carrier-signal long-distance cables. Such a cable may, for example, comprise a coaxial pair surrounded by a plurality of symmetrical wire pairs. Each of the, e.g., sixteen wire pairs forms a side circuit and respective pairs of side circuits are connected together each to form a spiral quad. By center-tapping the side-circuit transformers, the eight spiral quads are imparted phantom-circuit operation.
The eight phantom circuits, due to poor crosstalk characteristics as between the phantom and side circuits are typically utilized only for low-frequency transmissions, e.g., for low-frequency audio transmissions or as service lines.
When establishing a digital transmission network, it is desirable to use as the transmission medium the carrier-signal long-distance cables already present. It is possible to simultaneously transmit analog signals modulated onto a carrier on the side circuits and digital signals on the phantom circuits, in the same frequency range. However, then, due to the poor crosstalk characteristics of the cable, the carrier-modulated analog signals are subject to distortion attributable to the digital signals. One can avoid this problem, by transmitting the digital signals at a frequency not the same as, but instead higher than the frequency range utilized for transmission of the analog signal-modulated carrier signal. However, then a compromise must be found between, on the one hand, the increase of cable attenuation with increasing frequency and, on the other hand, the transmission-level decrease needed in the lower frequency range.
On account of the transmitting properties of carrier-signal cable of the type in question, satisfactory use of the side circuits for transmission of the analog signal-modulated carrier and of the phantom circuits for transmission of the digital signals (e.g., in accordance with data transmission scheme PCM 30D) becomes possible if the transmitted power density spectrum of a 2048 kbit/sec PCM signal is shifted out of the 0 to 2 MHz frequency range of the baseband into a frequency range of 1 to 3 MHz. If the converted-frequency PCM signal thusly employed is, for example, a pseudoternary digital signal, then such signal will exhibit a distinct spectral maximum at a frequency equal to one half the bit transmission rate of the original signal, i.e., a maximum at 1024 kbits/sec. A pseudoternary signal, it will be understood, is a signal capable of assuming three distinct values, but with its three values utilized merely to convey information represented or representable by only two values, i.e., a ternary signal used to represent the bits of a binary signal.
Such conversion of a binary signal into a pseudoternary digital signal can be implemented by means of amplitude modulation at one half the binary signal's bit transmission rate or frequency. Federal Republic of Germany published patent application DT-OS No. 23 39 806 (published Feb. 27, 1975) describes a circuit configuration for modulation of a pulse-code-modulated and pseudoternary-coded signal. The carrier employed is a rectangular pulse train having a bit repetition frequency derived from and equal to one half that of the binary signal. The carrier is derived from the bit repetition frequency of the binary signal by means of a frequency divider. The pseudoternary-coded signal is applied to the first terminal pair of a double push-pull modulator. The second terminal pair of the modulator receives the rectangular carrier signal, the latter having a pulse-duration/pulse-period ratio of 1:2 and one half the bit repetition frequency of the pseudoternary digital signal. The output signal of the modulator is likewise a pseudoternary-coded signal. This prior-art technique requires, as a preliminary, that the pseudoternary-coded input signal be split up into two unipolar pulse trains, one pulse train having only the positive amplitude values and the the pulse train only the negative amplitude values of the pseudoternary input signal. Also, it is necessary to derive from the rectangular carrier signal a further carrier signal which is the complement or logical inversion of the first one. Furthermore, the points where the pulses of the digital input signal and of the carrier signal each bit or leave zero values must be phase-shifted relative to each other by 90.degree., thus requiring means for maintaining a definite phase relationship between the digital input signal and the carrier signal. Last and not least, the use of a pseudoternary-coded signal as an input signal for such modulator requires means for converting the signal of actual interest, i.e., the original binary signal, into pseudoternary-coded form, in the first place.
Accordingly, the prior-art technique in question, which converts the frequency of the binary signal of interest to a more desirable frequency value by pseudoternary coding, involves considerable expense for implementing circuitry.